1. Field of the Invention
This invention relates to electro-optic communication systems in general and, more particularly, to circuits for driving light-emitting diodes or the like in those systems.
2. Description of the Prior Art
Light-emitting diode (LED) based data transmission system is a popular choice for low-cost data communication applications. However, LED based systems are not as high a speed as laser based systems. The primary reason for the disparity in speed between a laser and a LED based system is the relatively slow turn-on and turn-off time of the LED compared to a laser. The relatively slow turn-on and turn-off of the LED is substantially limited by the junction capacitance of the LED.
In U.S. Pat. No. 4,295,226, assigned to the same assignee as this invention, an LED driver is shown which shortens the turn-on and turn-off time of the LED. Because of the unidirectional nature of the LED, a small amount of current flows through the LED in the OFF state so that a negative current overdrive may be simulated. This speeds-up the turn-off of the LED. A consequence of this, however, is that the LED in the OFF state has sufficient current flowing therein to produce light. Hence, the contrast ratio between the ON and OFF states is less than the maximum possible and may be insufficient for certain applications.